The efficient conversion/storage of solar energy into chemical bond energy via the splitting of water into its elements—hydrogen (H2) and oxygen (O2)—represent a clean source of renewable fuel. Conventional electrolytic cells not only require a high pH, but also require operation at an overpotential that makes them unfeasible. A catalyst system may be used to reduce the overpotential to commercially practical levels. The catalyzed conversion of water into O2, protons (H+) into solution and electrons delivered to the protons can be used to make H2 or to chemically reduce other molecules including carbon dioxide (CO2). This technology can be applied in fuel cells for electricity production, and in electrolyzers and solar cells for production of O2, H2, and other hydrocarbon fuels. For example, a photoelectrochemical (PEC) cell or reverse fuel cell is a device for splitting water with energy from sunlight. The use of water as a source and sunlight as energy implies this technology is inherently sustainable and globally scalable, and could provide vast amounts of fuel (hydrogen), oxygen, and other hydrogenic precursors for reduction of carbon dioxide to hydrocarbon fuels from ordinary water.
Development of water oxidation catalysts to replace costly noble metals in commercial electrolyzers and solar fuel cells has been an unmet need preventing global development of hydrogen fuel technologies. Several metal oxides including IrO2 and RuO2 are already in use in industrial electrolyzers, but are made from rare and costly metals that are not globally scalable. Accordingly, there is a need for inexpensive electrodes made from earth-abundant elements.
Recent advances in methods for synthesizing transition metal oxide (TMO) nano-particles with the spinel structure in contact with proton conduction sites have produced more efficient catalysts for water oxidation that are suitable for renewable hydrogen production, when coupled with a proton reducing cathode. Such advances are applicable to energy storage problems inherent to intermittent solar energy conversion (i.e., photovoltaic (PV) and wind). One catalytic system capable of oxidizing water to molecular oxygen is the photosystem II water-oxidizing complex (PSII-WOC) found within photosynthetic organisms. PSII-WOC is expressed by the following equation (1):2H2O→O2+4H++4e−  (1)
The catalytic core of this enzyme is a CaMn4Ox cluster, which is conserved across all known species of oxygenic phototrophs. Many attempts to develop a biological water oxidation catalysts with a modest overpotential (E0=1.23 V at pH=0) have focused on Ru and Ir based compounds, which are inherently resource limited.
The chemical principles that govern the PSII-WOC, specifically the Mn—O bonding, have been studied through catalytic water oxidation capabilities of structurally related synthetic molecular manganese-oxo complexes. Patent Application Publication No. US 2010/0143811 discloses Mn4O4L6, where Mn4O4 is a manganese-oxo cubane core and L is a ligand stabilizing core such as (C6H5)2PO2 or MeO(C6H5)2PO2, as demonstrating catalytic activity. Recently, spinel-type Co3O4 nanoparticles have demonstrated catalytic capabilities. However, water oxidation activity by spinels has exhibited a strong dependence on crystallite size and surface area, frequently necessitating high overpotentials and alkaline conditions to accelerate the rate.
Accordingly, there is a need in the art for efficient water-oxidizing catalysts made from low-cost earth-abundant materials, particularly those used in connection with PECs. There also remains a need for a greater understanding of what limits the rate of turnover of reactants to products at photoelectrodes with complex three-dimensional architecture. Applicants have recognized a need for TMOs exhibiting high activities, simpler synthetic routes, and compatibility with PEC device fabrication. The present invention addresses these needs, among others.